Conversion of industrial wastes such as hazardous organic materials, sewage sludge, food processing by-products such as brewer's spent grain, and agricultural residues such as potato waste into useful products such as fuel or feedstock is increasing in importance. Newly developing conversion technologies require high performance heat exchangers and reaction vessels. Several biomass conversion technologies, described in U.S. Pat. No. 3,733,255 and in company literature of VerTech Treatment Systems, Denver, CO., process viscous slurries containing 5 to 35 percent solids, organic salts, and water at pressures of 1,000 to 5,000 psi and temperatures of 350 to 1000 degrees fahrenheit. Slurries having a low solids content or having solids which flow easily at ambient temperature can be moved through pipes by a pump. Slurries having a high solids content are usually difficult to move through pipes at ambient temperature. Sludges and slurries have a high Prandtl number, and viscosity of the slurries is also high at ambient temperatures. The high temperature and high pressure conditions are also usually corrosive. Heating such a slurry requires a scraped surface heat exchanger, which agitates the ambient temperature slurry in the heat exchanger, mixes the slurry and renews the surface film on the primary heat transfer surface, thereby overcoming the tendency of the slurry to stick to the heated surface. In addition, the heat exchanger must withstand corrosion, high temperature and pressure, and scraping.
Traditional heat exchanger design uses a single walled pressure vessel. A high pressure active seal is necessary to permit penetration of the rotating shaft of the scraper into the pressure vessel. These active seals operate by maintaining a thin film of water between rotating discs. A separate pump pressurizes the water film to a pressure greater than the operating pressure of the vessel to be sealed so that a continuous film of clean water leaks across the seal into the vessel. Few pressure vessel materials can meet all of the design requirements over the lifetime required for this type of heat exchanger, and active seals for this application cost from about $15,000 to about $25,000. In addition to their initial cost, inspection and maintenance of high pressure active seals is costly, and the performance and safety of this type of specially designed single-walled pressure vessel is difficult and expensive to assure.
All existing heat exchangers and reaction vessels used for thermochemical conversion of biomass have different pressures on the interior and exterior of the heat exchanger or reaction vessel, which necessitates the use of thick-walled pressure vessel and the use of a corrosion resistant material or a liner, and the use of active seals in cases where a scraper is needed.
It is therefore an object of the present invention to provide a dual-shell pressure balanced vessel capable of heating high Prandtl number sludges and slurries and capable of withstanding the high pressure and temperature operating conditions over the required life of the equipment.
It is a further object of the present invention to provide a dual-shell pressure balanced vessel that is inexpensive to construct yet efficiently accomplishes its purposes.
It is a further object of the present invention to provide a dual-shell pressure balanced vessel that is safe, and easy to operate and maintain.
It is a further object of the present invention to minimize the required thickness of corrosive resistant materials, and replace an active seal with a passive shaft seal where a scraper is needed.
It will be recognized that although the invention is directed toward industrial biomass waste conversion reaction vessels, it is applicable to any type of reactor or heat exchanger in which it is desired to have equal pressure on the interior and exterior of the heat exchanger or reaction vessel.